The present invention relates generally to a novel apparatus for mooring a vessel floating on the surface of a body of water to the floor or seabed beneath that body of water by means of a single point mooring. More particularly, the present invention relates to a fluid transfer buoy that comprises a frame with a load carrying shaft, a buoyancy chamber freely rotatable around the load carrying shaft, pipes and conduits for transfer of fluids or gasses to or from a vessel moored to the single point mooring, and catenary-type anchor arrangements to moor the apparatus permanently at an offshore location in the vicinity of the pipeline.
A common form of single point mooring is a buoy floating on the surface of a body of water and anchored at anchor points to the floor of that body of water with a number of anchor chains. It has been found that the number of anchor chains required for suitable mooring is minimally three but can be as many as twelve. A typical design is shown in U.S. Pat. No. 3,365,734. A flexible pipe terminates at the buoy and connects the buoy with the pipeline on the floor of the body of water. On top of the buoy there is mounted a turntable supported by a bearing which turntable rotates relative to the deck of the buoy. Mounted concentrically with the turntable is a fluid swivel which is of course in fluid communication with the terminus of the flexible pipe leading to the vessel at one end, and the seafloor pipeline at the other end. The combination of turntable and swivel can rotate freely about the vertical axis of the buoy to accommodate the motion of the moored ship in response to tide, wind and current.
With this known arrangement, the vessel is moored to the buoy with ropes, and a floating hose connects the fluid swivel on the turntable of the buoy with the vessel piping. With such an apparatus, a fluid connection between pipeline on the floor of the body of water and the vessel piping is established that allows free rotation of the vessel around the single point mooring for transfer of gases or liquids.
The prior art bearing arrangement between buoy and turntable is either by a set of bogie wheels running over a rail, or a single large-diameter roller bearing with three roller paths to resist against radial, axial and tipping moments. Since the turntable carries the floating hoses and mooring ropes, the turntable has to have extensions protruding over the buoy body usually at the water line. The rotating turntable typically dictates that the buoy have a circular shape; otherwise the fluid connections would overhang the edge of the buoy significantly in some positions.
The protruding overhanging turntable elements are susceptible to damage from the moored vessel, or from supply of work vessels. It is also difficult to provide an adequate fendering around the buoy, since this would require even larger and longer protruding parts. The overhanging character of the floating hose pipe connection makes the interconnection between the hose and the pipe cumbersome, since the maintenance crew cannot be provided with overhanging platforms or railings, and further since workmen must be constantly watchful of the relative movement of the turntable relative to the buoy deck.
Disadvantages exist in the prior art also with respect to the bearing arrangements which have been used. Bogie wheels are simple to repair, but the corrosive atmosphere in the splashzone area makes severe corrosion of the rail inevitable. This is not readily solved by using a corrosion-resisting material such as plastic for the rail, since these are generally too soft to act as a rail material. The continuous overrolling of the bogie wheels on the rail under load increases the corrosion effect. The acceptable load levels of bogie wheels over rails is limited, and in order to obtain sufficient wheels in contact with the rail to spread the load, a large rail diameter has to be chosen, preferably at or near circumference of the buoy. Buoy-kissing between vessel and buoy makes this arrangement very sensitive to deformation of the rail, which could hamper the rotation of the turntable.
A roller bearing with the same strength can have a much smaller diameter, since more roller bearings are carrying the load in a lubricated environment. The smaller diameter means the bearing may be centrally located on the buoy making it more safe against deformation in case of a severe collision between a vessel and the buoy. A difficulty presented by such a bearing is the protection against water ingress and corrosion. It is known that maintenance is often very difficult at exposed offshore locations, and good seal protection is almost impossible to realize on large diameters. In case of a turntable rotating over a buoy body, it is difficult to achieve a satisfactoy sea water seal between the turntable and the buoy deck to prevent intrusion of sea water into the bearing when rough seas cause waves to break over the buoy.